Characterization of transmembrane transport | EurekAlert!

image: Voltage ion transport through an ion redox transistor exhibits properties similar to those of a field-effect transistor, and the measured currents in the nanoparticle through the voltage-gated channel depend on the reduction potential applied to the membrane. Venkatesh group art.
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Credit: Beijing Zhongke Journal Publication Co. Ltd.

This study is led by Dr. Vijay Venkatesh (Department of Mechanical and Aerospace Engineering, The Ohio State University). Characterization of transmembrane ion transport across porous substrates at the nanoscale is critical for integrating membrane spacers in drug delivery, gate circuits and batteries. The current set of scanning probe microscopy techniques are limited to investigating ion transport as a surface phenomenon and do not provide insight into the underlying mechanisms of ion transport within the material under investigation. In a recent article, they presented “Surface scanning ion conduction microscopy” as a high-resolution microscopy technique for the quantitative characterization of transmembrane ion transport across porous substrates. The goal of this paper is to extend the above-mentioned technique to further their understanding of ion transport via an electrochemical arrangement referred to as an “ion-reduction transistor”. The ion redox transistor is a previously developed device To regulate transmembrane transport and mitigate thermal aberration in commercial Li-ion batteries.

Conducting polymers such as polypyrrole (PPy) doped with anions such as dodecylbenzene sulfonate (DBS)), polystyrene sulfonate (PSS) and dodecyl sulfate (DS .)) is an attractive candidate for regulating ion transport due to the introduction/expulsion of cations into/out of the polymer backbone during reduction/oxidation. The concept of using conductive polymers as an “ion gate” was first proposed by Murray and Burgmaier. It was observed that the membrane switches between the ON-OFF state during polymer reduction/oxidation, resulting in controlled ion transport. Price and colleagues extended this principle to develop ion transport systems for separating different metal ions from aqueous solutions. They show that applying a pulsed potential waveform to the conducting polymer results in higher ion flux. Misuska and colleagues investigated the permeability properties of bathcoprone isolephonic acid (BCS) doped polypyrol.) to transport metal ions such as Co2+he is2+zinc2+. To achieve pulsed drug delivery, Santini and colleagues made a porous microchip on which a thin layer of Au was sprayed. The application of an electric field expelled the chemicals through the porous channels in the device. Obedian and colleagues fabricated poly(3,4-ethylenedioxythiophene) nanotubes to transport dexamethasone as a function of the electrochemical state of the polymer. Jeon and co-workers synthesized a nanoporous membrane consisting of polypyrrole deposited on oxidized aluminum oxide to transport serum albumin isothiocyanate as a function of the redox state of the polymer.. It should be noted that the conductive polymer was deposited such that the equivalent pore size of the membrane increased/decreased when PPy was oxidized/decreased, resulting in greater/lower flux of ions across the nanopillars in the substrate. Recently, Hery and Sundaresan made an ion-reduction transistor consisting of polypyrrole spanning through the pores of a polycarbonate pathway etching membrane.. The application of an electric potential to the polymer resulted in a bidirectional transfer of Li+ ions across the porous membrane due to the jump pathways created in the polymer backbone. An ion redox transistor was used as a smart membrane separator in Li+ Battery to regulate ion transport at elevated temperatures and prevent heat escape.

Despite the above developments in the use of conductive polymers for various technologies such as gas filtration, gate channels and membrane separators, the transport of ions at the nanoscale in conducting polymers is poorly understood. Although there are a large number of research articles with transmembrane properties of synthetic membranes using ion conduction microscopy, the literature on in situ imaging of transmembrane ion transport via a membrane spacer that can regulate ionic current as a function of its electrochemical signature is limited to a report investigating the increase in Equivalent pore size of nanochannels deposited with PPy using atomic force microscopy (AFM). While AFM images of the core substrate depicted a change in equivalent pore size as a function of the redox state of the polymer, no information about the differences in ionic flux across the polymer membrane was revealed. Furthermore, Laszlo and colleagues report the need for progress in ionic conductivity microscopy, primarily for the purpose of distinguishing differences in ionic flux of conducting polymers from volumetric expansion due to ion ingress.

To address the need for an imaging technique that can quantitatively image ion transport across conductive polymers, this article uses surface scanning ion conduction microscopy using shear force (SF) imaging as a method to investigate ion transport kinetics in ionic redox transistors. It turns out that the application of downsampling to PPy (DBS) (VM) under constant transmembrane potential (VAC) facilitates entry of ions to redox sites in the polymer and drives transport across the membrane. The transmembrane current increases with the increase of the potential applied to PPy (DBS) (V.M), thus turning the transistor from the off state to the on state. An equivalent circuit model of the system was developed and it was shown that the membrane current was due to the increase in the conductivity of the polymer under a reducing voltage. Finally, surface scanning ion conductivity microscopy was used to determine topography and topography-related membrane transport across a set of pores. The increase in local membrane currents is attributed to a higher potential drop between PE and QRCE, and was measured using the modified GoldmanHodgkin-Katz (GHK) equation. Surface-tracked scanning ion conduction microscopy is expected to serve as a tool to characterize transmembrane ion transport across various ionic devices used in chemical separation, drug delivery by gas filtration, and desalination.

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